Recently, from the viewpoint of saving of resources and saving of energy, there is a mounting demand for reusable secondary batteries such as nickel-hydrogen, nickel-cadmium or lithium ion batteries. In particular, the lithium ion secondary battery is light in weight, high in electromotive force, and high in energy density. Owing to these features, the demand is expanding as driving power source for mobile telephone, digital camera, video camera, laptop computer, and various portable electronic appliances and mobile communication devices.
On the other hand, as the energy density tends to be higher in the downsizing trend, heat may be generated at high temperature depending on the manner of use. Accordingly, the safety is more and more important in the battery and the battery pack.
For example, when such battery is overcharged or overdischarged, or may be internally short-circuited or external short-circuited, gas may be generated inside. The generated gas causes to elevate the internal pressure of the battery. As a result, the outer jar may be ruptured by elevation of internal pressure. To prevent such rupture, these batteries are provided with exhaust hole or safety valves for releasing the gas.
In the battery pack having such batteries contained in a casing, a battery pack built in a structure capable of releasing the gas to outside of the casing has been propose for enhancing the safety.
Moreover, when such gas is generated, for example, due to effects of an overheated battery, smoke or fire may be induced in the battery pack in a worst case.
Hitherto, as disclosed in Japanese Unexamined Patent Publication No. 2006-228610 (patent document 1), the battery pack is proposed in a structure capable of preventing smoking or firing of the battery if the battery content ejects out. That is, in patent document 1, the battery pack includes a rechargeable battery, a filter unit, and an armor member. The filter unit is an outer part of the battery, and adsorbs the flammable matter contained in the content ejected from the battery. The armor member covers the battery and the filter unit, and is provided with a discharge hole for discharging the content purified by passing through the filter unit to outside.
The conventional battery pack of patent document 1 has such configuration, and if the content is ejected from any portion of the battery due to abnormality of protective circuit or the like, first the content is held within the armor member by the armor member covering the battery. Hence, the content will not leak out of the battery pack. The flammable matter contained in the content held within the armor member is securely adsorbed by the filter unit. Further, only the content not containing the flammable matter is discharged outside of the armor member through the discharge hole provided in the armor member. Thus, leak of the flammable matter to the surrounding of the battery pack is blocked, and smoking and firing can be prevented.
Or as disclosed in other prior art, in Japanese Unexamined Patent Publication No. 2006-185756 (patent document 2), the casing of the battery pack is made of a metal, and the battery pack enhanced in non-flammability of the casing is proposed.
However, in the case of patent document 1, a certain thickness is needed in the filter unit in order to adsorb sufficiently the content ejected from the battery, and the casing size is increased. Hence, such configuration of patent document 1 is contrary to the trend of compact design of battery pack.
In the case of patent document 2, a sufficient non-flammability is assured, but the casing is made of a metal. Hence, such configuration of patent document 2 is heavy in the weight of the battery pack, and when applied in a portable electronic appliance, it is contrary to the trend of weight reduction.